GB2267348A

GB2267348A - Electrochemical oxygen sensor

Info

Publication number: GB2267348A
Application number: GB9211125A
Authority: GB
Inventors: Walter John King
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-05-26
Filing date: 1992-05-26
Publication date: 1993-12-01
Also published as: GB9211125D0

Abstract

An oxygen sensor comprises two chambers communicating via a narrow electrolyte passage 11, a sensing electrode 2 in one chamber, a reference electrode 9 which may be in either chamber, and a counter electrode 8 in the other chamber. Oxygen being sensed passes through a diffusion membrane 4 to the sensing electrode 2. A potentiostat applies a small negative potential to the sensing electrode relative to the reference electrode. A current flowing externally between the counter electrode and the sensing electrode is a measure of the level of oxygen in the medium outside the membrane 4. The electrodes may be platinum and the electrolyte sulphuric acid. In operation, oxygen is converted to hydroxide ions at the sensing electrode. The hydroxide ions pass through the narrow, capillary passage 11 to the counter electrode 8 where they are reconverted to oxygen which diffuses out of the cell through membrane 10. <IMAGE>

Description

Sensor for Oxygen or other Gas This invention relates to a sensor for oxygen or other gas.

Oxygen sensors find wide use in industry, medicine and numerous other applications where continuous monitoring of oxygen level is necessary or desirable. Among the most popular types of oxygen sensor has been the electrochemical or galvanic type, which is relatively inexpensive to make and, because it is small and has a low power requirement, it is portable and convenient to use.

The galvanic oxygen sensor is frequently referred to as the Clark oxygen sensor, after its inventor. In its basic form, the Clark oxygen sensor comprises a cathode, usually of silver and acting as the oxygen sensing electrode, coupled galvanically to a consumable anode or counter electrode, also usually of silver: the two electrodes are in contact with an electrolyte of potassium chloride solution. The cathode or oxygen sensing electrode is covered by a membrane, through which oxygen diffuses from the exterior medium. The rate of oxygen diffusion is proportional to the partial pressure of the oxygen in the exterior medium, and so the electrical current generated by the cell is proportional to, and therefore provides a measure of, the amount of oxygen in the medium being monitored.

The main disadvantages of the Clark oxygen sensor are its short life and its requirement for frequent maintenance.

The lifetime is limited by the corrosion of the anode or counter electrode and, because the cell runs continuously, its lifetime is of the order of 6 to 12 months. Also the electrolyte is continuously consumed and needs changing at regular intervals: this process usually involves removal of the membrane, which is liable to be damaged in the process and require replacement.

I have now devised a sensor for oxygen or other selected gas, the sensor having a long life expectancy and being maintenance free, because the cell components are stable and are not consumed.

In accordance with this invention there is provided an electrochemical sensor, comprising two chambers communicating via a narrow electrolyte passage and having a sensing electrode in one chamber, a counter electrode in the other chamber, a reference electrode in either chamber, a diffusion barrier through which the sensing electrode is accessible from an exterior medium, means for applying a predetermined potential to the sensing electrode relative to the reference electrode, and means for monitoring an external current flow from the counter electrode to the sensing electrode.

In an oxygen sensor to be described herein, the sensing electrode is of a material (e.g. platinum) electrocatalytically sensitive to oxygen. Oxygen reaching the sensing electrode is reduced to hydroxide ions which then pass to the counter electrode, where the reverse electrochemical reaction takes place and oxygen is evolved. In effect, the sensor pumps oxygen from one chamber to the other, reducing it to hydroxide ions at the sensing electrode and evolving it from the hydroxide ions at the counter electrode, without any net gain or loss of mass. The narrow passage between the two chambers (e.g. a capillary) prevents the oxygen evolved at the counter electrode from passing back to the sensing electrode.

The sensor can be used in two broad types of oxygen measurement applications, either immersed in water or aqueous fluids to monitor the quantity of dissolved oxygen, or in air or gas to monitor the level of oxygen therein.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawing, the single figure of which is a diagrammatic section through one embodiment of gas sensor, in the form of an oxygen sensor, in accordance with this invention.

Referring to the drawing, there is shown an example of oxygen sensor, comprising a body 1 having upper and lower chambers communicating with each other through a capillary bore 11. In the upper chamber there is a cathode or sensing electrode 2, a diffusion membrane 3 over and in contact with the electrode 2, and an optional porous membrane 4 e.g. of PTFE over the diffusion membrane 3, for protection and to exclude water if the cell is to be immersed. An annular member 5 is clamped down onto the body 4 by means of fixing screws (not shown) and compresses a rubber sealing ring 6 against the membrane to make a seal, so that the only access to the sensing electrode 2 is by diffusion through the membrane 3.

The lower chamber houses a reference electrode 9, an anode or counter electrode 8, and a porous membrane 10, all secured in place by a twist washer 7 which fits tightly into the open side of the chamber. The porous membrane allows oxygen, generated at the anode or counter electrode 8, to escape from the lower chamber.

The capillary 11 is filled with electrolyte e.g.

sulphuric acid, preferably soaked into an inert absorbing material such as silica gel: the arrangement prevents significant quantities of oxygen passing into the upper chamber from the lower chamber.

Each of the three electrodes may comprise a porous plastics material such as PVC soaked in the electrolyte e.g.

sulphuric acid and coated on one side with platinum. The sensing electrode 2 would have its platinum side uppermost: the reference and counter electrodes 9,8 would be arranged so that their platinum sides do not contact each other, e.g. the reference electrode 9 may have its platinum side uppermost and the counter electrode 8 may have its platinum side lowermost.

Platinum connecting wires (not shown) extend from the exterior of the cell to the respective electrodes.

An external power source is connected between the sensing electrode 2 and the reference electrode 9 to apply a small negative potential to the sensing electrode 2 relative to the reference electrode 9, e.g. of 1 volt. This external source preferably comprises a potentiostat, which permits negligible current to flow through the reference electrode whilst maintaining the predetermined potential. The potentiostat circuit need only occupy a very small space and has a low power requirement.

A current flow is established between the counter electrode 8 and the sensing electrode 2 via an external circuit, the counter electrode 8 taking up a potential about 1 volt positive relative to the reference electrode 9. This current is proportional to the rate of diffusion of oxygen to the sensing electrode 2. An thermistor is included within the body 1 to measure the internal temperature of the cell and provide compensation for the measured current, since the rate of diffusion of oxygen through the membrane 3 increases with temperature. The signal from the thermistor may also be used to provide a temperature readout.

The sensing electrode 2 is electrocatalytically sensitive to oxygen. Hydroxide ions (OH-) are generated at a rate proportional to the rate of incoming oxygen, and these hydroxide ions pass through the capillary 11 to the counter electrode 8 acting as an anode, where the reverse electrochemical reaction takes place and oxygen is evolved: this oxygen diffuses out through the membrane 10. Thus, the cell in effect pumps oxygen from one chamber to the other without any net gain or loss of mass.

The reference electrode 9 may be disposed in the upper chamber, below the sensing electrode 2, instead of in the lower chamber.

The entire sensor can be housed in a probe e.g. for dissolved oxygen measurement, or within a hand-held instrument for gas sensing. Electrical connections between the sensor and instrumentation can provide for a continuous display of oxygen level, a temperature display, and other microprocessor generated functions such as time-weighted averages.

Claims

1) An electrochemical sensor, comprising two chambers communicating via a narrow electrolyte passage and having a sensing electrode in one chamber, a counter electrode in the other chamber, a reference electrode in either chamber, a diffusion barrier through which the sensing electrode is accessible from an exterior medium, means for applying a predetermined potential to the sensing electrode relative to the reference electrode, and means for monitoring an external current flow from the counter electrode to the sensing electrode.

2) A sensor as claimed in claim 1, in which the potential applying means comprises a potentiostat.

3) A sensor as claimed in claim 1 or 2, in which the sensing electrode comprises platinum.

4) A sensor as claimed in any preceding claim, in which the reference electrode comprises platinum.

5) A sensor as claimed in any preceding claim, in which the counter electrode comprises platinum.

6) A sensor as claimed in any preceding claim, in which the electrolyte comprises acid.

7) A sensor as claimed in claim 6, in which the electrolyte comprises sulphuric acid.

8) An electrochemical sensor substantially as herein described with reference to the accompanying drawing.